1) Field of the Invention
The present invention relates to an amplifier provided at an output stage of an intermediate frequency amplifying circuit of a mobile communication system. More specifically, the present invention relates to an amplifier provided at an output stage of an intermediate frequency amplifying circuit of a mobile communication system that is activated intermittently for reducing power drain on the battery.
2) Description of the Related Art
The automobile has become indispensable in contemporary life, and comfort and convenience when driving has become increasingly important. To this end, vehicles can be provided with a mobile communication system which can be used as a telephone system.
An automobile equipped with a mobile communication system, e.g., a mobile telephone system, is called a mobile station, and this mobile station can communicate not only with a fixed telephone such as a telephone in an office or at home but also with another mobile station. This communication is established by radio waves between the mobile station and a land station, and is transferred to a conventional telephone system at the land station.
In a typical analog mobile communication system, a user of the system occupies one frequency for transmission and another frequency for receiving to communicate. This system is called a frequency-division multiple access (FDMA) in a mobile communication system.
Incidentally, in the mobile communication system, a service area of one land station that is called a zone is limited, and according to the movement of the mobile station from a certain zone to another zone, the mobile station has to change communication channels by changing the communication frequency. Accordingly, in the mobile communication system, when the mobile station is communicating by using one channel at a certain zone, the mobile station intermittently detects the RSSI (received signal strength indicator) levels of the other channels in neighboring zones one by one. Note that, the useable channels in the neighboring zones are previously determined by the system. The detected RSSI levels of the other channels in the neighboring zones are reported to the land station to which the mobile station is presently communicating, and the reported RSSI levels are monitored by the land station. As a result, when the mobile station moves to another zone, a suitable channel having the highest RSSI level at the other zone is selected by the land station, and the switching of the communication channel is executed.
In the mobile communication system, a stand-by period of the mobile station is generally far longer than a communicating period thereof. The stand-by period means the time period that the mobile station is waiting for a call send or a call receive operation. Accordingly, in the mobile communication system, only the circuits necessary for receiving data and detecting a call send or a call receive operation from the received data are activated intermittently to reduce power consumption from the battery during the stand-by period. And, when the call send or the call receive operation is detected from the received data, the other circuits necessary for communicating are activated.
An intermediate frequency amplifying circuit and an active filter connected to the intermediate frequency amplifying circuit via a capacitor are the part of the circuits necessary during stand-by. And due to the existence of the capacitor between the intermediate frequency amplifying circuit and the active filter, the rise time characteristics of the input signal to the active filter become worse, so that the output signal from the active filter is delayed and the detection of a call send or a call receive operation is also delayed.
As a countermeasure to this delay, a rise time speed-up circuit is provided in series with the capacitor. However, providing the speed-up circuit causes an increase in the number of circuit elements and an increase in cost.